The human spine is configured to maintain an upright posture and, as such, must be capable of supporting substantial weight. Injuries or deformities of the spine may compromise this function. Orthopedic surgery to treat such conditions will often insert implants, for example a metal rod connected to sequential vertebrae via bone screws, to fix the position of the vertebrae and thereby stabilize the spine and augment support.
The first generation of vertebral screws could be connected to a rod in a single orientation (“monoaxial screws”). Variations in anatomy and difficulties in screw placement would frequently result in non-linear placement of screws, so that bending of a spanning rod or the use of special adaptors would often be required to provide a connectable configuration. Such intra-operative reconfiguration can be difficult and is error-prone.
Accordingly, a second generation of vertebral screws was developed which could connect with a rod in multiple orientations (“polyaxial screws”). For example, the fixation element portion of the screw could be pivotally joined to a housing, so that the housing could be oriented to connect with a rod, and then be fixed in place. Examples of patents relating to polyaxial screws include U.S. Pat. Nos. 6,280,442, 6,485,491, U.S. Patent Application Publication No. 2003/0004512, and U.S. Patent Application Publication No. 2001/0001119.
One problem encountered in the use of polyaxial screws is that the housings (or “heads”) of most polyaxial pedicle screws are freely movable with respect to the bone screw portion prior to rod insertion and tightening. A surgeon may rotate the housing of the pedicle screw to line up for rod placement, but, once the housing is released, it may move and shift, causing rod placement to be difficult.
The present invention addresses this problem, in that, in one aspect, it provides for polyaxial screws in which the housing is frictionally held to the bone screw. This frictional joint allows the surgeon to place the pedicle screw in the bone and move the housing relative to the bone screw to accept the rod such that once the housing is moved to accept rod placement, it will substantially maintain its position. This feature is especially advantageous for endoscopic pedicle screw placement, where assembly is performed in a confined space. In exemplary embodiments of the invention, the head to bone screw friction can be adjusted to the desired amount. This minimizes tolerances needed during manufacturing and minimizes manufacturing scrap inherent in other designs.